seperate channel after fork

QNX DEVELOPERS General Programming
1

Hello,

I wrote a programm which forks two or more processes. each process creates a channel and a timer.Each process should now wait till its coresponding timer elapsed and a pulse is received. the problem is that the call to ChannelCreate() returns the same channelID to all processes. Is this a typical behavior? I was thinking that each process would crate its own ChannelID. how do i get two seperate channel IDs in to seperate Processes?

//function will be called in a while(1) after fork()
inline int set_AfterActivate(double* after, double all, double until, double during, long* exec_counter)
{
	#ifdef __QNX_
	static struct sigevent  event;      // event to deliver
	static int    coid;      			 // connection back to ourselves
	static struct itimerspec   timer;      // the timer data structure
	static MessageT msg;           // the message itself
	static timer_t             timerid;    // timer ID for timer
	static int rcvid;              // process ID of the sender
	static int     chid;               // channel ID (global)
	 // Task wurde noch nicht ausgeführt Setze timmervariablen und befülle sie mit after und all
	 if((*exec_counter) <= 0){
		 //set Timer 
		if ((chid = ChannelCreate (0)) == -1) { //return the same ID in two seperate processes
			perror (NULL);
			exit (EXIT_FAILURE);
		}
		// create a connection back to ourselves
		coid = ConnectAttach (0, 0, chid, 0, 0);
		if (coid == -1) {
			perror (NULL);
			exit (EXIT_FAILURE);
		}
		// set up the kind of event that we want to deliver -- a pulse
		SIGEV_PULSE_INIT (&event, coid, SIGEV_PULSE_PRIO_INHERIT, CODE_TIMER, 0);
					  
		// First time execution create timer
	    if (timer_create (CLOCK_REALTIME, &event, &timerid) == -1) {
			perror (NULL);
			exit (EXIT_FAILURE);
		}	 
		timer.it_value.tv_sec = (long)((*after)/(double)MILLION);
		timer.it_value.tv_nsec = (long)((*after) * (double)TOUSEND);
		timer.it_interval.tv_sec = (((int)all)/MILLION);
		timer.it_interval.tv_nsec = ((int)all) * TOUSEND;
		//start timer
		timer_settime (timerid, 0, &timer, NULL);
		
		//if after not used exit function 
		if( ((int)(*after)) == UNUSED || ((int)(*after)) == 0 ) {
			(*exec_counter)++;
			return 0;
		}
		else{ //wait for pulse
			rcvid = MsgReceive(chid, &msg, sizeof(msg), NULL);
			(*exec_counter)++;
			return 0;
		}
	 }
2

This is normal.

Each process does have its own set of channelId’s. They do however all start at the same number. This is similar to how each process has it’s own set of open file id’s but they all start at the same number (stdin, stdout, stderr use 0, 1, 2 and if you do an fopen() call the returned fd will be identical across multiple processes).

If you want to send a message/pulse to a specific process you do that in the ConnectAttach() call by specifying a non-zero pid value in argument 2 (a 0 says you are connecting to your own channel).

// Connecting to yourself
coid = ConnectAttach (0, 0, chid, 0, 0);

// Connecting to someone else (could still be yourself if you specify your own pid)
coid = ConnectAttach (0, pid, chid, 0, 0);

Tim